我正在创建编号从＃1＃2＃3 ....开始的发票，但是我不知道如何开始。这是我想要的结构：

Invoice#1
|- info...
Invoice#2
|- info...
Invoice#3
|- info...

DatabaseReference ref 
 = FirebaseDatabase.getInstance().getReference().child("Invoice#" {increment}); //how to put increment here? if I declare i=1 in myactivity.java,it only runs in one

这是我拥有的发票模型的一部分

public class Invoice {
    String buyer,seller,typeOfPymt;
    int total;
    String billdate;
    int invoice_num;
    int discount;
    int tax;
    int grand_total;


    public Invoice(String buyer,String seller,int grand_total,String billdate,int invoice_num,int discount) {
        this.buyer = buyer;
        this.seller = seller;
        this.billdate = billdate;
        this.invoice_num = invoice_num;
        this.discount = discount;
        this.grand_total = grand_total;
    }

如何在模型内部添加增量？